A certain amount of air content can improve the workability of concrete, improve its cohesiveness and make the mixture difficult to separate; but the existence of air bubbles will correspondingly reduce the unit bearing area of concrete and reduce the strength of concrete to a certain extent, so we expect the air content of concrete to remain in a reasonable range [68] 。 From Table 5.6, it can be seen that the air content of C60 high strength concrete mixed by twin-screw mixer is lower than that of twin-horizontal mixer with blade holes under different mixing processes. According to the previous analysis, the air content of C60 high-strength concrete mixing by double helix mixer is low, so in practical engineering, when mixing C60 high-strength concrete with double helix mixer, the air content of concrete can be maintained between 2% and 3% by adding air-entraining agent, thus greatly improving the high-strength coagulation. The quality of soil.

As shown in Fig. 5.9, the relative errors of mortar density of two kinds of mixers under three different mixing processes show the same changing law: the relative errors of mortar density of high-strength concrete mixing by first mixing cement mortar method are the smallest; the high-strength concrete mixing by one feeding method is the largest; and the mining method is the largest. The law of wrapping sand and stone with cement is in the middle of the two. It can be seen that the mixing quality of high strength concrete can be effectively improved by choosing suitable secondary mixing technology without considering the mixer type. This is because secondary mixing technology can realize the gradient characteristics of water cement ratio in the interface transition layer which can not be achieved by traditional technology, and make the water cement ratio of slurry in the transition zone between aggregates. The distance from aggregate surface varies regularly with independent variables: the interface transition zone is weak and strengthened; the cement paste body is strong and weakened slightly, which improves the micro-uniformity of concrete interface; the second mixing process can also reduce the aggregation of cement particles, improve the dispersion of cement particles and increase the hydration degree. In addition, the secondary mixing process blocked the channel from free water to aggregate interface to a certain extent, improved the distribution of hydration products in concrete interface area, and comprehensively improved the performance of concrete. At the same time, the whole mixing process of cement-wrapped sand-stone method contains coarse aggregate, which is not conducive to high-speed mixing, and has a certain impact on the hydration reaction of materials, the rapid mixing and uniform distribution of cement mortar, so it will have certain disadvantages compared with the first mixing cement mortar method.

The macro-homogeneity index of concrete mixing with double helical shaft mixer is qualified when the single-feeding method is used, but the macro-homogeneity index of concrete mixing with double horizontal shaft mixer with holes in blades is not qualified. This shows that under the present test condition, the double horizontal shaft mixer with holes in blades is not suitable for single-feeding. C60 high-strength concrete is stirred by mixing method. In actual industrial production, we can obtain high strength concrete by adding mineral admixtures or prolonging the mixing time appropriately.

When using the first mixing cement mortar method, the macro homogeneity index (M and G) of the two mixers are the smallest under the three mixing processes, which shows that the two mixers can mix high strength concrete with better macro homogeneity under this mixing process. At the same time, it can be seen that M and G of concrete mixer with double helix axle are much lower than those of double horizontal axle mixer with blade holes, and the relative error of mortar density of mixer with double helix axle mixer B1 is the smallest, which is only 22.8% (test B2) of double horizontal axle mixer with blade holes under the same test conditions. It has great advantages to mix C60 high-strength concrete with twin-screw mixer under various working conditions.

The macro-homogeneity index of high-strength concrete mixing by two kinds of mixers is qualified when using cement-wrapped sand-gravel method, and the M and G of high-strength concrete mixing by double helix mixer are smaller than that of double horizontal axle mixer with blade holes, but the average compressive strength and the deviation coefficient of compressive strength of concrete in 7d are slightly worse at this time. It shows that under this mixing technology, the double helix shaft mixer has advantages and disadvantages in mixing concrete homogeneity and the double horizontal shaft mixer with holes in blades.

From Fig. 5.10, it can be seen that when C60 high strength concrete is mixed by one-time feeding method and first mixing cement mortar method, the strength of concrete mixing by double helical shaft mixer is higher than that by double horizontal shaft mixer with blade holes, and the strength of concrete mixing by first mixing cement mortar method reaches the maximum of the three mixing processes. 。 When using cement-wrapped sand-gravel method, the strength of concrete mixing by double horizontal shaft mixer with holes in blades is the largest among the three mixing processes, and is higher than that by double helical shaft mixer. This shows that the micro-homogeneity of concrete mixer with blade holes is better than that of double helix mixer under the condition of cement-wrapped sand-stone method. Similar rules can also be obtained for the deviation coefficient of compressive strength, in which the deviation coefficient of compressive strength of mixtures of test B1 twin-screw mixer is 0.014.

Minimum, only 43.8% (test B2) of the double horizontal shaft mixer with blade holes under the same test conditions.

Through the above analysis, the following conclusions can be drawn:

(1) Under different mixing processes, the two mixers mix C60 high-strength concrete at a relatively stable level, and the concrete air content of the double helix mixer is low.

(2) When using one-time feeding method and first-mixing cement mortar method, the concrete produced by the double helix mixer is far superior to that of the double horizontal axle mixer with holes on blades in both macro and micro homogeneity, and the high strength concrete with the best homogeneity under three mixing processes is obtained under the condition of first-mixing cement mortar method. The homogeneity of C60 high-strength concrete mixed by two kinds of mixers is similar when using cement-wrapped sand-stone method.

Summary

1. Through three stages of experiments, the optimum parameters of influencing factors for mixing C60 high-strength concrete with twin-screw mixer under three different mixing processes are obtained. The mixing speed is 1.6m/s and the total mixing time is about 90s, which is 25% shorter than the 120 s required in the actual site production. It can be seen that the double helix mixer can mix the C60 high strength concrete in a relatively short time, thus improving the mixing efficiency, reducing the mixing energy consumption, and in a certain degree. The service life of the mixer is prolonged.

2. When mixing C60 high-strength concrete with double helix mixer, the high-strength concrete produced by mixing cement mortar first has better homogeneity.

3. The air content of concrete is affected by the mix ratio of concrete. The air content of concrete under different mixing technology is basically kept between 2.0% and 3.0%, and the first mixing method is better.

4. The comparative test results between the twin-screw mixer and the twin-horizontal-axis mixer with holes in the blade show that the C60 high-strength concrete mixing by the twin-screw mixer has better concrete air content under different mixing technologies; when using the one-time feeding method and the first-mixing method, the concrete mixing device produced by the twin-screw mixer It has better homogeneity, and the method of mixing cement mortar first is the best; when using cement-wrapped sand-stone method, the homogeneity of C60 high-strength concrete mixing by two mixers is equal; while when using one-time feeding method, the macrohomogeneity index of concrete mixing by double horizontal axle mixer with holes in blades is not qualified and can not satisfy the homogeneity mixing. The standard of condensate. Therefore, under the condition of this test, it is suitable to mix C60 high strength concrete with double spiral shaft mixer.